1. Field of the Invention
This invention relates to cameras and, more particularly, to cameras having a constituent element made of a shape-memory material thermally deformable to restore the memorized shape.
2. Description of the Related Art
Of the shape-memory materials thermally deformable to restore the memorized shape, the shape-memory alloy, when heated by electric current flowing therethrough, can deform (for example, contract) to restore the memorized shape at a very fast speed. In the past, therefore, a wide variety of devices utilizing such a, property have been proposed.
In the application of this shape-memory material to cameras, however, the following drawbacks arise.
(i) The use of a drive source of the shape-memory material instead of an electric motor, though contributing to a simplification of the structure of the camera, if different drive members made of the shape-memory material are used for the focusing and exposure mechanisms, does not always lead to a great advantage of low cost.
(ii) The shape-memory material when heated above the transformation temperature rapidly restores to the memorized shape. But, after the supply of electric current for heating has been cut off, it takes a long time to get to the initial state by plastic deformation under the action of an external force (e.g., a spring or the like). Therefore, the camera cannot recycle the operation in a short time, and a phalographic chance will be missed.
Meanwhile, to improve this drawback, the aforesaid external force, for example, the bias force of a spring, may be very strongly acted on. For this case, however, the increased resistance during the restoring of the memorized shape (by contracting, for example) on heating invites problems of slowing down the contracting speed and decreasing the amount of displacement.
(iii) If the heating of the shape-memory alloy is controlled by the current supply control circuit, the temperature to which it is heated is influenced by the ambient temperature, varying from situation to situation. To make a safe design for the camera employing the shape-memory alloy, its workable conditions must be restricted to a very narrow range.